Increasingly, active organic molecules are used in electronic devices. These active organic molecules have electronic or electro-radiative properties including electroluminescence. Electronic devices that incorporate organic active materials may be used to convert electrical energy into radiation and may include a light-emitting diode, light-emitting diode display, or diode laser.
Two methods are commonly used to prepare organic light-emitting diode (“OLED”) displays: vacuum deposition, and liquid-phase processing. (The latter includes all forms of applying the layers from a fluid, as a true solution or a suspension.) Vacuum deposition equipment typically has very high investment costs, and inferior material utilization (high operating costs), so liquid-phase processing is preferred, especially for large area displays.
Liquid-phase processing for the deposition of organic active layers include any number of technologies for control of layer thickness on a substrate. Some of these technologies include self regulated methods to control thickness, including spin coating, rod coating, dip coating, roll coating, gravure coating or printing, lithographic or flexographic printing, screen coating etc. Other of these technologies seek to control deposition thickness using controlled deposition techniques including ink jet printing, spray coating, nozzle coating, slot die coating, curtain coating, bar or slide coating, etc.
Self regulated techniques suffer a number of drawbacks. Fluids used in coating OLED displays are often applied over surfaces with topography—electrodes, contact pads, thin film transistors, pixel wells formed from photoresists, cathode separator structures, etc. The uniformity of the wet layer deposited by a self regulated technique depends on the coating gap and resulting pressure distribution, so variations in the substrate topography result in undesirable variations in the wet coating thickness. Self regulated techniques generally suffer higher operating costs in that not all the fluid presented to the substrate is deposited. Some fluid is either recycled in the fluid bath (e.g., dip coating), or on the applicator (e.g., roll or gravure coating), or, it is wasted (e.g., spin coating). Solvent evaporates from the recycled fluid, requiring adjustment to maintain process stability. Wasting material, and recycling and adjusting material, adds cost.
Controlled techniques can provide lower operating costs. However, in some cases, continuous printing can lead to deposition of printing material outside the desired locations on a designated substrate. In one mode, the continuous liquid-phase printing material can break up into undesired droplets and deposit in undesired locations on the substrate. This defect is often referred to as splatter, and can cause performance degradation of the electronic device.
Previous techniques to address this problem involved collection of excessive printing material in a duct or channel and removal using an exhaust air flow. This solution has proved ineffective to eliminate the splatter defect. Another technique involved maintaining a collection surface at the same gap, or distance, between nozzle and surface to be printed. This maintains the continuous stream, or pillar, of the printing material. However, this solution causes puddling of the stream, or droplets upon reversal of the nozzle(s) in changing direction to make additional passes along the substrate. Solutions to the splatter issue include methods to reduce or eliminate the degradation of the pillar of printing material into aerosol size droplets, and additionally, eliminate puddling upon reversal of the nozzle(s) at end of printing pass.
Inconsistent formation of organic layers typically leads to poor device performance and poor yield in device fabricating processes. There continues to be a need for improved processes for the liquid-phase deposition of organic materials for OLED applications.